Former middleweight world titleholder Billy Joe Saunders is moving up to super middleweight and will face Shefat Isufi for a vacant world title, promoter Frank Warren announced on Monday.

The fight, which will stream live on ESPN+ in the United States, is scheduled to take place on April 13 at the SSE Wembley Arena in London. The fight was announced as being for the WBO's 168-pound title, which has been held for nearly three years by Gilberto "Zurdo" Ramirez, but it is unclear why the belt is vacant.

Carl Moretti, vice president of Top Rank, which promotes Mexico's Ramirez (39-0, 25 KOs), told ESPN after Warren's announcement that Ramirez has not vacated the title and they also have not received any correspondence from the WBO saying that he had been stripped.

Ramirez told ESPN earlier this month that he intends to fight his next bout this spring at light heavyweight but left the door open to return to super middleweight to defend his title after that fight. Ramirez is also up to date with his mandatory defense obligations.

Nonetheless, Saunders, who fought his last bout on Dec. 22 at 178 pounds, is leaving the 160-pound division, where he won a title in 2015 and made three defenses.

"It is a brilliant opening for me," Saunders said of the opportunity to go for another belt. "You've got these rumors that (middleweight titlist Demetrius) Andrade is going for shoulder surgery, as well as the financial side of it with the split. It all doesn't add up. Me winning this super middleweight title will put me in a great position for unifications with the likes of Callum Smith, while we've got Chris Eubank boxing James DeGale (on Saturday).

"There are a lot of big, big fights at this weight, but my real home of homes is middleweight, so I can move down at any time I want and be back as mandatory for my middleweight title and then become a three-time world champion."

Saunders was due to face Andrade, then his mandatory challenger, on Oct. 20 in Boston but the fight was canceled when Saunders failed a random Voluntary Anti-Doping Association urine test conducted on Aug. 30. He tested positive for the powerful banned stimulant oxilofrine in results that came back on Sept. 26. On Oct. 9, the Massachusetts State Athletic Commission denied Saunders' application for a boxing license because of the positive test.

Knowing he would be stripped of the title by the WBO, Saunders, who said the positive test was the result of using "a common decongestant nasal spray," relinquished the belt, and the fight with Andrade -- for which Saunders would have earned a career-high $2.3 million -- was canceled.

Andrade wound up facing late replacement Walter Kautondokwa instead, knocking him down four times and winning a near-shutout decision to win the vacant title.

Saunders, a southpaw, later had his six-month suspension lifted early by the WBO on Jan. 22 and was made Andrade's mandatory challenger. But Saunders (27-0, 13 KOs), 29, of England, will forgo that fight to instead face the obscure Isufi (27-3-2, 20 KOs), 29, a Serbia native fighting out of Germany. Isufi has been a pro since 2010 and is the bigger man but he has never faced a recognizable opponent.

"I am really looking forward to it and Isufi comes as a really stiff test. He is very, very strong and you have to be really switched on and calculating with this sort of opponent," Saunders said. "He is one of those who, with what he does, he does very, very well when the bell goes. He has boxed at light heavyweight and cruiserweight so he is going to be a big, big lump and I need to make sure I am on it and stay away from those big bombs."

Warren said that there will be major fights for Saunders at super middleweight if he defeats Isufi.

"He has operated at middleweight for a long time and reached the pinnacle, so we both now believe the time is right for him to step up and prove himself at super middleweight," Warren said. "If he is successful against Isufi -- and nobody is taking the threat he presents lightly -- the possibilities are huge, with unifications and great fights all out there for him."

Elsewhere Britain's Nicola Adams has been forced to call off her world title fight against Mexico's Arely Mucino scheduled for March 8 in London.

Adams has picked up an injury in training and the match will be rescheduled for later in the year.